use 5.10.0;
package Animal;
sub speak {
   my $self = shift;
   ref $self and $self = ref $self;
   say "$self says: ", $self->word;
}

package Cow;
@Cow::ISA = qw(Animal);
sub word { "moo" }

package Dog;
@Cow::ISA = qw(Animal);
sub word { "wuff" }

package main;

Cow->speak;
Dog->speak;
__END__
Cow says: moo
Dog says: wuff